1. Field of the Invention
The present invention relates to a method of forming a layer of thin film.
2. Description of the Prior Art
A hitherto known multi-color electrophotographic copying machine is typically constructed such that an electrostatically charged image is formed on a photosensitive screen with a large number of pores (preferably mesh-shaped pores) formed thereon, and it is then transferred onto an electrically chargeable layer (for instance, a photosensitive layer on a photosensitive drum) while a stream of ion beams (for instance, plus ion particles) is properly controlled.
FIG. 1 schematically illustrates a conventional two-color electrophotographic copying machine for reproducing a two-colored image on an original including a black colored image area and a red colored image area on the white paper surface. The illustrated machine includes a reciprocable original holding board 41 at the upper end part thereof and an original on the original holding board 41 is exposed to a light beam emitted from lighting lamps 42. Reference numerals 43 and 44 each designate a mirror, respectively, reference numeral 45 a stationary lens, reference numeral 46 a movable red filter adapted to protrude into and retract from the passage of a light beam and reference numeral 47 a movable dichroic filter adapted to protrude into and retract from the passage of a light beam. The dichroic filter 47 reflects red colored light therefrom but allows cyan colored light complementary to red colored light to pass therethrough. FIG. 1 illustrates the working condition of the machine in which the red color filter 46 is retracted from the passage of a light beam and the dichroic filter 47 is protruded into the passage of the light beam. A photosensitive body 53 in the form of a drum has a photosensitive layer 18 formed over its peripheral surface and the photosensitive layer 18 is uniformly charged by a corona charger 24 as the photosensitive body 53 is rotated in the clockwise direction. The photosensitive layer 18 is made of selen, organic semiconductor or like material.
In the proximity of the photosensitive body 53 are arranged a charger 54 for uniformly charging the photosensitive layer 18, a black color developing device 48 containing therein plus charged black toner, a red color developing device 49 containing therein plus charged red toner and a cleaning device 30 for removing residual toner and electric charge on the photosensitive layer 18. Reference numeral 31 designates a transferring drum having the same diameter as that of the photosensitive body 53. The transferring drum 31 rotates in operative association with the photosensitive body 53 while coming into rolling contact with the photosensitive layer 18 of the latter, or is rotated in the anti-clockwise direction. Reference numeral 63 designates a corona discharger or transferring electrode, reference numeral 32 a feed paper tray on which a number of papers to be electrophotographically copied are placed one above another, reference numeral 33 a paper feed roller for feeding copying papers 52 in the feed paper tray 32 one by one, reference numeral 34 a first pair of conveying rollers for conveying copying papers toward the transferring drum 31, reference numeral 35 an electrostatic separator for discharging electric charge after completion of electrophotographic copying operation to facilitate separation of copying paper from the transferring drum 31 and reference numeral 36 a separating pawl for forcibly separating copying paper from the transferring drum 31. Further, reference numeral 37 designates a fixing device with a heater incorporated therein. It should be noted that in practice a guide plate is provided in the machine but it is not shown in the drawing for the purpose of simplification of illustration.
On the other hand, a cylindrical photosensitive screen drum 17 is disposed at a position located outwardly of the photosensitive layer 18 in such a manner that its photoconductive layer is located opposite to the photosensitive layer 18. The screen drum 17 is arranged so as to rotate in the anti-clockwise direction in synchronization with the original holding board 41 and the photosensitive layer 18. Further, in the proximity of the screen drum 17 are arranged a screen charger 28, a screen discharger 39 comprising an electroluminescence plate or alternate current corona discharger to remove residual electric charge on the screen drum 17 and an electrically charged particle source 19 (corona discharger) disposed inside the screen drum 17 at a position opposite to the photosensitive body 53 to emit electrically charged particles therefrom.
As is apparent from FIG. 2, the photosensitive screen drum 17 has a large number of very fine pores 10 formed thereon and comprises a drum-shaped electroconductive screen base 11, one surface of which is exposed to the outside, an electric insulating layer 13 disposed at least on the other surface of the electroconductive screen base 11 (and on the wall surface of the pore 10 in the illustrated case), an electroconductive film 14 made of aluminum or like material disposed on the electric insulating layer 13 for the purpose of biasing, a photoconductive (photosensitive) layer 15 and an electric charge transporting layer 16. Specifically, the photoconductive screen base 11 is made of mesh of metallic material such as stainless steel, aluminum or the like material, the electric insulating layer 13 is made of polyethylene or like material and both the photosensitive layer 15 and the electric charge transporting layer 16 are made of organic semiconductor.
When the photosensitive screen 17 is made, the photoconductive layer 14 constituting the screen 17 is formed by using an aluminum vaporizing source 20 from which aluminum vapor is produced, as illustrated in FIG. 3. It should be noted that some part of the aluminum vapor 21 coming up from the vaporizing source 20 as identified by reference numeral 21' flows upwardly at a certain inclination angle relative to the screen base 11 in the direction as identified by arrow marks in the drawing with the result that it tends to pass through the pores 10 to reach the reverse side of the screen base 11 as seen from the vaporizing source 20 or it is deposited directly on the reverse side of the screen base 11. This causes the photoconductive layer 14 to be in electric communication with the screen base 11 (in the form of short circuit) via the aluminum part thus deposited on the reverse side of the screen base 11 as illustrated in FIG. 4, resulting in the photoconductive layer 14 failing to function properly. In view of the malfunction described above there has been hitherto proposed a countermeasure that a comparatively thin part 14a of the aluminum film 14 deposited on the screen base 11 (in particular, a thin part deposited on the edge portion 11a of the screen base 11) is removed by burning it with the use of electric current so as to assure that any short circuit to the screen base 11 is not formed. This leads to problems, however, in that an extra step for removing the thin part as mentioned above is required, the aluminum film 14 is damaged to some extent by the burning operation and the withstand voltage ability of the aluminum film 14 is reduced.